1. Field of the Invention
The present invention relates to a semiconductor device and a manufacturing method thereof, and more particularly, to a semiconductor device having an extra capacitor structure and a manufacturing method thereof.
2. Description of the Prior Art
Power semiconductor devices are mainly applied to power management devices, such as switching power supplies, power management integrated circuits for computers or computer accessories, backlight power supplies and motor control devices, etc., and can be sorted as metal-oxide-semiconductor thin film transistor (MOSFET) devices and insulated gate bipolar transistor (IGBT) devices, etc. In general, the power semiconductor devices are designed with trenches to lower power consumptions.
However, electronic products are developed toward lighter, thinner, shorter, and smaller designs, so that sizes and gaps of the trench MOSFET devices are continuously scaled down to have high integration and high density. In regard to trench NMOSFET device, when a width of each trench and a distance between the adjacent trenches are reduced, a coupling area between a gate conductive layer and an N-epitaxial layer respectively serving as a gate electrode and a drain electrode of the trench NMOSFET device is also reduced, and a contact area between a P-doped body region and an N-epitaxial layer is reduced as well. Accordingly, a capacitor between a gate and a drain of the trench NMOSFET device and a capacitor between a source and the drain of the trench NMOSFET device are reduced. As a result, an output capacitance of the NMOSFET device composed of the capacitor between the gate and the drain and the capacitor between the source and the drain is reduced, in which the capacitance between the source and the drain is much larger than the capacitance between the gate and the drain.
The trench NMOSFET device can be applied to a converter of a power management circuit, such as a switching device in a synchronous buck converter, and thus is switched on/off frequently. When the trench NMOSFET device is switched off, an output capacitor of the trench NMOSFET device is charged to have a voltage equal to a voltage of an external transformer. However, the converter further includes an inductor device, so that the output capacitor and the inductor device constitute an LC resonant circuit when the trench NMOSFET device is switched off, and a voltage spike is accordingly generated. Moreover, the output capacitance of the trench NMOSFET device is reduced with the decrease of the whole size of the device as well as the widths of the trenches. Accordingly, the voltage spike is increased when the trench NMOSFET device is switched off so as to generate higher power loss.
To prevent the voltage spike from being increased, a snubber circuit is generally utilized to electrically connect a source electrode and a drain electrode of a trench NMOSFET device 10 in parallel, and the snubber circuit is composed of a capacitor and a resistor electrically connected to each other in series. Please refer to FIG. 1, which schematically illustrates a circuit of reducing the voltage spike according the prior art. As shown in FIG. 1, a snubber circuit 12 is electrically connected between the source electrode S and the drain electrode D of a trench NMOSFET device 10 in parallel, and the snubber circuit 12 is composed of a capacitor C and a resistor R electrically connected to each other in series. Accordingly, a capacitor disposed outside the trench NMOSFET device 10 can be used to increase an output capacitance of the trench NMOSFET device 10 so as to reduce the voltage spike. However, an extra circuit device increases not only extra circuit costs but also extra fabrication costs due to extra welding process of.
As a result, it is still needed for a novel method of manufacturing a power semiconductor device to solve the aforementioned problem of the voltage spike in a simple and economic way.